This invention relates to an improved technique for coating silicate glass surfaces with an antireflection (AR) layer, i.e., for reducing specular surface reflection, especially for surfaces used in high energy laser systems.
It has long been known that the reflectance of light from glass surfaces can be reduced by chemical modification. Such techniques remove leachable components from the glass, leaving behind a porous skeletal layer of a depth approximately 1/4 the wavelength of the light to be transmitted, or an odd multiple thereof. Virtually all such chemical treatments have utilized complex acidic solutions and procedures optimized for one particular type of glass. Early work was performed almost entirely on optical glasses containing large amounts of lead or barium. Glasses containing only silica, alkaline earth metals, alkali metals and/or boron oxide (soda-lime and borosilicates) were not considered suitable for treatment. These processes used acidic solutions to remove the heavy metal ions (which modify the refractive index) by exchange of protons for the heavy ions. In almost all cases, the resulting surface was only moderately effective and specular reflectance could only be reduced by 50 percent. Further reduction in reflectance was effected by immersing the glass sample in a second bath containing HF or fluorides. (U.S. Pat. No. 2,348,704).
A later process using acidic solutions (U.S. Pat. Nos. 2,486,431 and 2,490,662) utilized complex silica saturated solutions of fluosilicic acid to produce antireflective surfaces on soda-lime and optical crown glasses. However, the process did not provide efficient reduction of reflectance for borosilicate compositions. Additionally, the complex nature of the process resulted in a lack of reproducibility and the chemical instability of the resultant surfaces hindered its practical utility.
A more recent process utilizes the difference in chemical reactivity of two different phases in heat treated borosilicate glasses of specific and limited compositions to produce leached antireflective layers using acidic solutions (U.S. Pat. No. 4,019,884). While possessing a very broad wavelength range of antireflective behavior, the process has the major disadvantage of requiring borosilicate glasses of relatively low chemical resistance. These are not conventional optical glasses. Normal optical glasses cannot be treated by this process. The nature of this phase separation process also causes significant light scattering effects in both the antireflective surface layer and in the bulk glass, further limiting its utility. In addition the AR coatings require careful handling.
All of these processes are disadvantageous in that they require acid treatments. Furthermore, they are highly limited by poor reproducibility, poor surface durability and/or low optical quality, etc. All are inapplicable to relatively large optical surfaces since inevitably unevenness is introduced which changes the optical figure.
The only subtractive chemical treatment process utilizing neutral or nearly neutral solutions was described by Schroeder. See West German Patent Nos. 821,828 and 964,095, British Pat. No. 698,831 and Schroeder, Intern. Cong. on Glass, Vol. 8, pp. 118-123 (1974). However, as for all the classical etch/leach processes, the Schroeder method has been found by the art to be highly disadvantageous and not applicable to situations having stringent requirements, e.g., optical uses. These negative findings include poor reproducibility, low optical quality, uneven application on large articles, sensitivity to contamination, mechanical instability of the layers, low durability, inter alia. Thus, they have never been employed practically or industrially. For specific comments in this regard, see Lowdermilk et al., Laser Focus, December 1980, p. 64-70, especially pp. 64 and 68; Elmer et al, Ceramic Bulletin 58, No. 11, 1979, pp. 1092-1097, especially p. 1092, Lowdermilk et al, Appl. Phys. Lett. 36(11), 1980, pp. 891-893, especially p. 891; and Holland, "The Properties of Glass Surfaces," Chapman & Hall, London 1966, p. 155; whose disclosures are incorporated by reference herein.
Furthermore, in laser applications, especially high energy laser applications, provision of non-reflective surfaces is especially critical and difficult. For some high energy laser systems, the AR surfaces are the feature limiting the achievement of higher power densities. Current methods include the coating of optical elements with antireflective added-on layers, e.g., based on interference phenomena. However, these layers are quite limited in their resistance to higher power levels and are useful over only a relatively narrow bandwidth. In addition the phase separated glasses mentioned above have been utilized. See, e.g., Lowdermilk et al, Lowdermilk et al and Elmer et al, supra. However, all these approaches have the significant disadvantages discussed above and in these references.
Thus, the problem exists to provide a method for rendering antireflective a wide range of optical surfaces in terms of shape and composition, and in such a manner that the resultant surfaces are useful for a broad range of optical applications.